1. Field of the Invention
This invention relates to a method of manufacturing a silicon epitaxial wafer having a silicon epitaxial layer formed by vapor phase growth on a silicon single crystal substrate doped with a relatively high concentration of boron.
2. Description of the Related Art
Silicon epitaxial wafers, having a silicon epitaxial layer grown in vapor phase on a silicon single crystal substrate manufactured by the Czochralski method (simply referred to as the CZ method, hereinafter), while being doped with a high concentration of boron so as to adjust the resistivity to as low as 0.02 Ω·cm or below (referred to as p+CZ substrate, hereinafter), have widely been used, for example, for the purpose of preventing latch-up, and of eliminating any defects from a device forming region.
The p+CZ substrate has a large number of oxygen precipitation nuclei formed in a crystal pulling process over a period from solidification of the crystal to cooling down to room temperature. Size of the oxygen precipitation nuclei is generally as very small as 1 nm or below. The oxygen precipitation nuclei can grow up to oxygen precipitate, if the nuclei are kept at a temperature not lower than a nuclei forming temperature and not higher than a certain critical temperature relevant to re-solid solubility into the bulk of silicon single crystal. The oxygen precipitate is a sort of BMD (bulk micro-defect) causative of deterioration of withstand voltage and current leakage, so that it is desired to be formed as less as possible in the device forming region. The oxygen precipitate can, however, effectively be used as a getter for heavy metal components in a device manufacturing process, in regions of the substrate not destined for device formation, so that it has been a general practice to intentionally form the oxygen precipitate in the silicon single crystal used for growth in the manufacture of silicon epitaxial wafers, only to a degree not causative of nonconformities such as warping. The effect of gettering heavy metals by the oxygen precipitate is one of so-called IG (intrinsic gettering) effects.
By the way, the oxygen precipitation nuclei are known to re-solid-solubilize themselves into the bulk of silicon single crystal and disappear, if the substrate is kept at a temperature higher than the above-described critical temperature. As for the silicon epitaxial wafers, vapor phase growth process of the silicon epitaxial layer corresponds to high temperature annealing at 1,100° C. or above where the nuclei disappear, and this means that the oxygen precipitation nuclei, resided abundantly before the vapor phase growth, can largely be reduced due to thermal history during the vapor phase growth. Reduction in the oxygen precipitation nuclei suppresses formation of the oxygen precipitate in the process of manufacturing semiconductor devices, even if the initial oxygen concentration in the adopted silicon single crystal substrate is high, and achieving the IG effect is less hopeful.
Aiming at solving this problem, there is proposed a method of subjecting a silicon epitaxial wafer to low temperature annealing at 450° C. to 750° C., both ends inclusive, so as to newly produce the oxygen precipitation nuclei in the p+CZ substrate, and then to middle-temperature annealing (a temperature range between those for low-temperature annealing and high-temperature annealing) to thereby grow the oxygen precipitate (Japanese Laid-Open Patent Publication Nos. H9-283529, H10-270455, and International Patent Disclosure WO01/056071).
Japanese Laid-Open Patent Publication No. H9-283529 describes, in the section titled “Preferred Embodiments of the Invention”, that low-temperature annealing aimed at forming the oxygen precipitation nuclei in an oxygen atmosphere results in formation of silicon oxide film on the surface of the silicon epitaxial wafer. Thus-formed unnecessary silicon oxide film can be removed by hydrofluoric acid cleaning, as is well known in the art. Removal of the silicon oxide film by the hydrofluoric acid cleaning, however, raises the particle level on the surface of the silicon epitaxial wafer after the cleaning. Adoption of the hydrofluoric acid cleaning aimed at removing the silicon oxide film after the low-temperature annealing also increases the number of process steps, and consequently increases costs for manufacturing the silicon epitaxial wafers.
It is therefore a subject of this invention to provide a method of manufacturing a silicon epitaxial wafer, on the premise that annealing for forming oxygen precipitation nuclei is carried out in an oxidizing atmosphere, capable of suppressing the final residual thickness of the silicon oxide film formed during the annealing to a level equivalent to that of native oxide film without adopting hydrofluoric acid cleaning, and furthermore suppressing increase in the particles after the cleaning.